Abstract/Summary

Generalisation is a process of reducing the volume of information while preserving its significance. It is required in order to maintain legibility on reducing the scale of a map. Many maps are digitised at 1:10 000 scale under the BGS Digital Map Production System, and computer methods are required to assist in generalising to 1:50 000 scale. The procedure is described in a data flow diagram.
Analysis of the rationale and procedures of geological mapping throws light on possible long-term changes to take advantage of new technology. Focussing on the decisions and procedures of generalisation clarifies the difficulties of developing an expert system. Neither approach leads to automation because the machine lacks the necessary background knowledge of geological processes and of human reasoning and visual perception.
An alternative is to develop interactive graphical procedures on the computer which support the geologist and cartographer in formulating and implementing decisions on generalisation. While this “amplified intelligence” approach relies totally on judgments made by the human expert, the computer assists with tedious operational tasks. It is a transitional approach, which should increase understanding of the complex decisions, and encourage acquisition of more fully structured knowledge, leading to further automation.
Present methods of manual generalisation cause the map to be less informative than it could be, and possibly misleading in unpredictable ways. The survey-scale map is itself a generalisation of the geologists’ observations and interpretation. New technology offers unique opportunities for rationalising and improving map design and presentation.
Generalisation differs from statistical sampling in concentrating on presentation rather than on facilitating logical inference. Statistical sampling requires the underpinning of a formalised model. The digital geological spatial model (DGSM) can support more rigorous data collection than the map, and is therefore seen as the future key to a more reproducible, representative and testable representation of the real world geology.
The current emphasis, however, has to be on digitising existing geological maps, particularly at 1:10 000 and 1:50 000 scale. The two map series provide distinct but overlapping input to the DGSM, where they share the same structure and logical data model, but must be separately identified, to ensure that each is internally consistent. Where individual items have counterparts at both scales, the linkages should be identified, to encourage overall consistency.
A step by step approach to generalisation procedures within the DGSM is recommended, aiming to consolidate and build on current best practice and to incorporate new developments only after their value has been established by discussion and prototyping. Because many types of presentation can be generated from the DGSM, it is necessary to separate scientific generalisation, reflected within the DGSM, from cartographic generalisation, which should be deferred until the map is produced.
The systems currently supporting the DGSM are complex and expensive, and therefore restricted to expert users. They can generate a range of digital images which could be made more widely available, and could be handled with general purpose software. Techniques of digital data compression with information loss, which are currently an area of high investment and rapid progress world-wide, overlap with cartographic generalisation, and could be applied to such images for changing scale or display device.